Process and apparatus for liquefying low-boiling gases



Aug. 11, 1964 H. KOEHN ETAL PROCESS AND APPARATUS FOR LIQUEFYING LOW-BOILING GASES Filed May 51. 1960 5 Sheets-Sheet 1 COMPRESSOR COMPRESSOR IN VE N TORS HELMUT KOEHN LYLE J. LA PLANTE RICHARD L. SHANER RUDOLPH F'. STENGEL 4 ATTORNEY Aug. 1954 H. KOEHN ETAL PROCESS AND APPARATUS FOR LIQUEFYING LOW-BOILING GASES 5 Sheets-Sheet 2 Filed May 31, 1960 COMPRESSOR COMPRESSOR (L-64 Z Z w INVENTORS HELMUT KOEHN 0" LYLE J. LA PLANTE RICHARD L.SHANER RUDOLPH F STENGEL ATTORNFV Aug. 11, 1964 H. KOEHN ETAL PROCESS AND APPARATUS FOR LIQUEFYING LOW-BOILING GASES Filed May 31, 1960 5 Sheets-Sheet 5 Go: \fiaqzuv as: 522: ME 8 EEzw ooom 8mm ooom 82 89 com Om 3 IUJOOUMEOm Om OJOU uP mIEumDmuO KUWZUOZOU KNJOOUQDQ 55:03

INVENTORS HELMUT KOEHN LYLE J LA PLANTE RICHARD L. SHANER RUDOLPH F. STENGEI.

A T TORNEY A g- 1964 H. KOEHN ETAL PROCESS AND APPARATUS FOR LIQUEFYING LOW-BOILING GASES Filed May 31, 1960 5 Sheets-Sheet 4 COMPRESSOR COMPREJJQR lNVENTORS HELMUT KOEHN .9 LYLE J.L.A PLANTE RICHARD L.SHANER RUDOLPH F.STENGEL V v B W W ATTORNEY Aug. 11, 1964 H. KOEHN ETAL PROCESS AND APPARATUS FOR LIQUEFYING LOW-BOILING GASES Filed May 31, 1960 5 Sheets-Sheet 5 COMPR$SOR COMPRESSOR IN VE N TORS L ERQ TEN NNE NAAT. HLHS E 0A KL M D T P U ML u m YmU HLRR A T TORNE Y United States Patent PROCESS AND APPARATUS FOR LIQUEFYING LOW-BOILING GASES Helmet Koehn, Tonawanda, Lyle J. La Plante, Grand Island, Richard L. Shaner, Williamsville, and Rudolph F. Stengel, Buffalo, N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed May 31, 1960, Ser. No. 32,974 23 Claims. (Cl. 629) This invention relates to an improved process of and apparatus for cooling and liquefying low-boiling gases, and more particularly to an improved refrigeration system for liquefying feed gases having boiling points below -80 C.

The consumption pattern of low-boiling industrial gases such as oxygen and nitrogen has changed appreciably in recent years, and created a pressing need for an economical system to liquefy such low-boiling material when available in the gas phase. This is due mainly to the fact that it is usually more economical to store and transport such low-boiling gases in the liquid phase instead of the gas phase. Also, it is sometimes desired to employ low-boiling gases in liquid form for very cold refrigerativc purposes, such as in food preservation or for cryogenie research requirements, and for fuels in rocket engines.

The prior art has proposed and employed numerous systems for cooling and liquefying low-boiling gases, but all of these systems have important disadvantages or limitations. For example, the power consumption of certain prior art liquefier systems is prohibitively high, and in others an inordinately large number of expensive heat exchangers are required. Also, some systems utilize high pressure equipment which is relatively complicated and difiicult to maintain and control automatically, and in addition have limited rangeability.

It is the principal object of the present invention to provide a process and apparatus for cooling and liquefying low-boiling gases, the steps and apparatus being arranged and related so that each step is conducted under substantially ideal or optimum conditions, the overall result being a highly efficient and low cost system. Another object is to provide a process and apparatus for cooling and liquefying low-boiling gases in which the refrigerant gas and the feed gas are compressed to relatively low pressures and are of the same composition group.

Still another object is to provide a process and apparatus for cooling and liquefying low-boiling gases which utilizes inexpensive low pressure heat exchangers and rotary type compressors and expanders, and which is both compact in physical size and reliable in operation.

These and other objects and advantages of this invention will be apparent from the following description and accompanying drawing in which:

FIG. 1 shows a fiow diagram of a system for cooling and liquefying low-boiling gases, according to the present invention;

FIG. 2 shows a flow diagram of a system similar to FIG. 1 but modified to include two stages of work expansion;

FIG. 3 is a graph showing the relationship between fluid temperature differences and product fluid enthalpy for one and two stages of refrigerant gas work expansion;

FIG. 4 is a flow diagram of another system for cooling and liquefying low-boiling gases according to this invention, wherein both the feed and refrigerant gases are compressed in the same machine; and

FIG. 5 is a flow diagram of still another system wherein the feed and refrigerant gases are compressed to an intermediate pressure in the same machine, and further compressed in separate booster units.

Patented Aug. 11, 1964 Briefly, the invention contemplates a refrigeration system for liquefying feed gas by heat exchange with a refrigerant gas, the two gases having boiling points below C. at atmospheric pressure. The feed gas is compressed to a pressure of at least 70 p.s.i.g., partially cooled in a first cooling step, further cooled in a second cooling step, liquefied and withdrawn as a liquid product. The refrigerant gas is also compressed to a pressure of at least 70 p.s.i.g., aftercooled to a temperature below about 40 C. and work expanded to sufficiently low pressure to develop power and cool the gas to a temperature below the condensation temperature of the feed gas. The work expanded refrigerant gas is consecutively passed through the liquefaction, further cooling and first cooling steps in countercurrent heat exchange with the feed stream to effect the cooling and liquefaction thereof. The warmed refrigerant is withdrawn from the warm end of the first cooling step and recirculated to the refrigerant compression step.

A critical feature of this invention is that the refrigerating fluid is maintained in the gaseous state throughout its circulation, in marked contrast to previously proposed systems for liquefying low-boiling gases wherein the refrigerating medium i condensed and reevaporated. It has been found that the gas refrigeration feature affords unexpected advantages in the form of lower pressures, all rotary compressors and Work expanders instead of reciprocating machines, simpler control, and less expensive heat exchangers. For example, a two-phase refrigeration system necessitates higher pressures to permit recovery of sufficient refrigeration by virtue of Joule- Thompson throttling. These higher pressures necessitate the use of reciprocating compressors and Work expanders, With their attendant control problems and lubricant contamination of the fluid being processed. Rotary machines eliminate these difficulties. Finally, a less expensive liquefier and liquefier control system may be provided since only one instead of two fluid phase changes occurs therein.

The power arising from the work expansion is preferably transferred directly to the refrigerant compression step at the highest pressure level thereof, but alternatively may be absorbed by other means such as electrically if desired. For example, if the refrigerant compression step includes a first compressor and a booster compressor, the expansion turbine is preferably directly connected with the booster compressor since this provides for the most efficient transfer of the power. That is, the high shaft speeds which permit the most efficient and economic design of the work expansion turbine can be most effectively utilized to absorb the available power by centrifugally compressing an equivalent mass, higher density gas stream at higher pressure and smaller volumes, rather than compressing a large volume gas stream at lower pressure, such as occurs in the first stage of compression.

This invention is particularly suitable for processing nitrogen and oxygen as feed streams, and will be described in detail with respect to these components. It is to be understood, however, that it may be advantageously employed for liquefying any low-boiling gas having a boiling point below about -80 C. For example, with a suitable refrigerant gas it is also applicable to ethane, ethylene, methane, argon, fluorine, carbon monoxide, neon, hydrogen and helium.

The preferred refrigerants are nitrogen and air, that is, a refrigerant wherein nitrogen is at least the principal constituent. However, other refrigerants may be used depending on the feed gas composition. The refrigerant gas may be of the perfect gas type, not relying upon special nonideal properties to provide refrigeration at certain temperature levels by Joule-Thompson throttling.

a.) For example, when liquefying hydrocarbon gases such as methane, methane could also be advantageously used as the refrigerant gas.

Referring now more specifically to FIG. 1, two completely separate feed gas circuits are illustrated, so that for example, oxygen and nitrogen streams may be separately processed and liquefied. If only one feed gas component is to be processed, a portion of the flow may be processed in each circuit, or alternatively, only one circuit may be employed. The cycle will be initially described in terms of oxygen feed gas passing through both circuits. The oxygen feed gas, supplied to conduits and 11, is compressed to at least 70 p.s.i.g., and preferably to about 150 p.s.i.g. in compressors 12 and 13, respectively. This pressure level is necessary to permit subsequent liquefaction of the feed gas. The compressed oxygen feed gas is then passed through conduits 14 and 15 to aftercoolers and lubricant traps (not shown), and next through respective shutoff valves 14a and 15a to warm leg heat exchanger 16 as a first cooling step to about 46 C. The feed gas flows through passageways 17 and 18, and is cooled by countercurrently flowing refrigerant gas in passageway 19.

The partially cooled oxygen feed gas is discharged from warm leg heat exchanger 16 into conduits 20 and 21 and preferably directed to externally refrigerated forecooler 22 for cooling therein to about 60 C. That is, the partially cooled oxygen feed gas in conduits 20 and 21 is directed to passageways 23 and 24, respectively, in forecooler 22, and countercurrently cooled by an externally supplied refrigerant flowing through passageway 25. The preferred external refrigerant is dichlorodifluoromethane, although monochlorodifluoromethane, ammonia or nitrogen are also suitable. It is to be understood that the externally refrigerated forecooling step is preferred but is not essential to the present invention, and that the necessary cooling may alternatively be effected in warm leg heat exchanger 16 and cold leg heat exchanger 26.

The forecooled oxygen feed gas is discharged from forecooler 22 into conduits 27 and 28, hence to cold leg heat exchanger 26 for further cooling in passageways 29 and 30, respectively, by heat exchanging with the countercurrently flowing refrigerant in conduit 31. The further cooled oxygen gas is discharged from cold leg heat exchanger 26 at a temperature of about 140 C. into conduits 32 and 33, and directed to liquefier 34 for flow through communicating passageways 35 and 36, respectively, and liquefaction by countercurrently flowing gaseous refrigerant in passageway 37. In liquefier 34, the oxygen feed stream is cooled to saturation, totally condensed, and the product liquid is preferably subcooled to a temperature of about 186 C. It is withdrawn as a pressurized liquid product through conduits 38 and 39, and passed through control valves 40 and 41, respectively, to storage means or consuming means as desired. One reason for subcooling the liquid product is to avoid flashoff on expansion to a storage tank, preferably at a pressure of 0-15 p.s.i.g. Any vapor generated from the product liquids downstream of control valves 40 and 41 from the pressure reducing step is preferably separated from the liquid in separate vessels 71 and 72, and returned through respective conduits 73 and 74 to feed gas conduits 11 and 10 for reprocessing. The remaining low pressure liquid product is withdrawn from separators 71 and 72 through conduits 75 and 76, respectively. It is to be understood however, that the liquid product may alternatively be stored at substantially the feed stream pressure if desired.

It is necessary to transfer either warmed compressed feed gas or subcooled product liquid from one circuit to the other for any purpose such as to control the proportion of total feed gas processed in each passage, appropriate interconnections and valving means may be provided. For example, interconnecting conduits 42 and 43 with shutoff valves 44, and 4-5, respectively, may be provided at the warm end of the heat exchange system. Also, interconnecting conduits 46 and 47 with shutoff valves 48 and 49, respectively, plus shutoff valves 50 and 51 in conduits 38 and 39, respectively, may be provided at the cold end of such system.

All control of the two feed stream pressures is effected at the cold end of the liquefier by automatic control valves 40 and 41. All other valves in both the warm and cold ends, such as 44, 45, 48, 49, 50, and 51 are used for flow balancing or shutoff purposes to divert the flow wherever desired, such as into a particular storage tank or other further usage.

Clean, dry nitrogen may, for example, be supplied at about 8 p.s.i.g. and 15 C. in conduit 50a with control valve 50b therein, and pressurized in compressor 51, preferably of the centrifugal type, to a pressure of at least 50 p.s.i.g. and preferably about p.s.i.g. Alternatively refrigerant gas inlet flow may be effected by inlet guide vanes (not shown) inside compressor 51a, instead of by valve 50b. The compressed nitrogen refrigerant gas is discharged into conduit 52 and aftercoolcd in passageway 53 to a temperature below about 40 C. by heat exchange with a suitable fluid such as water in thermally associated passageway 54. The aftercooled nitrogen gas is then further compressed in the turbine loading booster compressor 55 to a pressure of at least 70 p.s.i.g. and preferably about p.s.i.g., and discharged therefrom into conduit 56. The further compressed nitrogen gas is then aftercooled in passageway 57 again to a temperature below about 40 C. by heat exchange with an appropriate fluid such as water in thermally associated passageway 58.

The further compressed, aftercooled nitrogen is first directed to the warm end of warm leg heat exchanger 16 for cooling therein to about 46 C. by flow through passageway 57 in countercurrent heat exchange relation with the refrigerant in passageway 19. The partially cooled, compressed nitrogen gas is then directed through conduit 58 to passageway 59 in forecooler 22 for further cooling therein to about 60 C. However, as previously disussed, the forecooling step is not essential to this invention. The forecooled compressed nitrogen is then discharged into passageway 60 and directed to the warm end of cold leg heat exchanger 26 for flow through passageway 61 in countercurrent heat exchange relation with the refrigerant in passageway 31.

The compressed nitrogen gas is cooled in cold leg 26 to a temperature of about 141 C., and discharged into conduit 62 for flow to a work expander such as turbine 63. At this point, the nitrogen is expanded to a low pressure preferably in the range of 6-10 p.s.i.g., although the discharge may be at subatmospheric pressure if desired to lower the condensing temperatures. However as previously discussed, liquefaction of the refrigerant gas is purposely avoided to prevent reduced efliciency and possible erosion of the expander parts due to its handling mixed liquid-vapor flow, and to avoid two-phase flow in the heat exchangers with the resulting additional equipment such as entrainment separators, liquid levels, and the like. The nitrogen gas is cooled to about 187" C. by virtue of such work expansion, and the power developed in the expansion turbine is preferably transferred directly to the highest pressure level of the refrigerant compression step. This is preferably accomplished by employing shaft 64 to connect turbine 63 with booster compressor 55, to provide highly efficient transfer of the available power. As previously discussed, high shaft speeds which permit the most eflicient and economic design of the turbine, can be most effectively utilized to absorb the power by centrifugally compressing an equivalent mass of higher density gas stream at higher pressures and smaller volumes, rather than compressing a large volume gas stream at lower pressure, such as occurs in the first stage of compression. Alternatively, at least part of the work expander power may be absorbed by other means such as an electric generator (not illustrated), and used for reducing the net power requirements of the cycle.

The work expanded nitrogen is discharged from turbine 63 into conduit 65, and passed from the cold end to the warm end of the feed gas heat exchange system to refrigerate the latter. More specifically, the work expanded nitrogen is first passed to the cold end of liquefier 34 for flow through passageway 37, thereby desuperheating, condensing and preferably subcooling the product oxygen stream in thermally associated passageways 35 and 36. The nitrogen is simultaneously warmed to about -156 C. and thereafter directed through connecting conduit 66 to passageway 31 of cold leg heat exchanger 26. for further cooling of the partially cooled oxygen feed streams. Finally, the partially rewarmed nitrogen refrigerant gas is directed through communicating conduit 67 to warm leg heat exchanger 16 and passageway 19 therein for warming to near ambient temperature.

The resulting warmed nitrogen refrigerant gas is discharged from the heat exchange system through conduit 68 and recirculated to connecting conduit St? for return to the inlet side of compressor 51. Makeup nitrogen gas from a suitable source is admitted to conduit 50 through conduit 69 and control valve 70 therein to overcome system losses through compressor seals, and the like. Any vapor arising downstream of control valves 40 or 41 from the pressure reducing step therein may be separated in separators 71 and 72 and returned through conduits 73 and 74- to feed conduits 11 and 10, respectively, for reprocessing.

The FIG. 1 system also may be employed to liquefy nitrogen instead of oxygen feed gas. It is to be noted that because of the difference in the normal boiling points of oxgen and nitrogen, it is necessary to compress nitrogen to a higher pressure than oxygen to obtain the same cycle efiiciencies. As explained previously, the feed gas stream must always be provided at sufiicient pressure to permit liquefaction by the lowest temperature level attained by the refrigerant stream.

If both oxygen and nitrogen feed gas liquefaction is desired, another passageway may be employed in the warm and cold leg heat exchangers 16 and 26, and forecooler 22, or each feed gas constituent may be directed through one of the existing, illustrated circuits.

For a particular combination of refrigerant and feed gas, optimum performance is obtained by carefully selecting the feed gas pressure which in combination with the refrigerant gas pressure and recirculation rate will provide small temperature differences Within the heat exchangers and also permit maximum utilization of external forecooling, if employed. The effect of increasing the condensing pressure of the feed gas stream is to reduce its latent heat and increase the degree of subcooling required. Thus, the feed gas pressure is selected to maintain optimum economy between the latent heat and subcooling requirements for providing a liquid stream preferably at essentially ambient pressure, as will be understood by those skilled in the art.

It has been found that a temperature pinch occurs in the liquefier heat exchanger 34 at the point where condensation begins. That is, the temperature of the product feed stream being cooled in passageways 35 and 36 is reduced at that point to very nearly the temperature of the expanded recycle refrigerant stream flowing countercurrently to it in passageway 37. If the refrigerant recirculation ratio is reduced to about 7.2 c.f.h. (NTP) nitrogen circulated per 1 c.f.h. (NTP) oxygen liquefied, using external forecooling to 60 C., this temperature pinch becomes so severe as to limit the utilization of any additional refrigeration from the forecooler. However, optimum performance (with forecooling to 60 C.) is obtained with a recirculation ratio of about 8.5 c.f.h. (NTP) nitrogen recirculated per 1 c.f.h. (NTP) oxygen liquefied and subcooled. This ratio opens the temperature difference at the pinch point to about 6 C., and also opens the temperature difference at the warm end of liquefier 34 to about 16 C. to achieve eificient liquefier operation. While a recirculation ratio greater than 8.5 may be used, it results in more refrigeration being made available at the lowest temperature level than can be eifectively utilized. This causes increased temperature differences within the various heat exchangers and also permits less external fore cooling to be used, thus causing reduced overall cycle eficiency.

During the startup and cooldown phase of operation of the present liquefier, a condition will arise whereby the power developed by work expander 63 will exceed that which can be absorbed in booster compressor 55, resulting in overspeeding of the latter. To alleviate this problem, bypass conduit 77 containing control valve 78 may be provided between conduit 62 processing the compressed and cooled nitrogen gas, and conduit 66 transporting the partially warmed work expanded nitrogen gas at the warm end of liquefier 34-. A suflicient quantity of gas is diverted from conduit 62 to conduit 66 to maintain the desired energy balance between Work ex pander 63 and booster compressor 55. Alternatively, this bypass line and valve may be located at the warm end of the heat exchange system between conduits 56' and 68. However, if expander 63 is loaded by an electric generator to maintain essentially constant speed, the bypass conduit is not advantageous.

In FIG. 2, another embodiment of this invention is illustrated in which two work expansion steps are employed instead of one step. In the one step embodiment illustrated in FIG. I, nearly all of the low temperature refrigeration is supplied by the Work expansion and the optional but inexpensive forecooling can only be employed to a limited extent as an additional source of refrigeration. This is a limitation when additional refrigeration is needed as for example when nitrogen is employed both as the feed gas and the refrigerant gas. When oxygen is supplied as the feed gas and nitrogen is used only as the refrigerant gas, their difference in boiling points may be advantageously employed in the liquefier since the oxygen feed gas is at substantially higher pressure than the work expanded nitrogen refrigerant gas. However, when the feed gas and refrigerant gas have the same chemical composition the advantage of dilferent boiling points is lost, and additional low temperature refrigeration must be supplied if all of the feed gas is to be liquefied. One solution to this problem is raising the feed gas inlet pressure, for example from p.s.i.g. to 300 p.s.i.g. This presents serious disadvantages as for example higher power requirements and more expensive heat exchangers.

The present invention solves this problem in an unexpected and highly eflicient manner by employing two work expansion steps arranged in series, and passing the partially expanded refrigerant gas in heat exchange with the condensing feed fluid in the liquefier. The rewarmed partially expanded refrigerant gas is then further expanded to the final low pressure and again passed in heat exchange with the condensing feed fluid in the liquefier. Thus, the same recirculated quantity of cold refrigerant gas is twice available for cooling the condensing feed fluid in the liquefier, and additional low temperature refrigeration is supplied.

Another advantage of the two-stage embodiment is that the temperature pattern within the other heat exchangers is shifted so that greater advantage may be taken of the preferred forecooling step utilizing an external refrigerant fluid. More specifically the cooling range of the forecooling step may be broadened from about 25 C. to 14 C., so that the forecooler carries about one-third of the refrigeration load.

Referring now more specifically to FIG. 2, the differences from the PKG. 1 embodiment will be described in detail, the two systems being similar in all other respects.

The compressed nitrogen gas cooled in cold leg 26 to a temperature of about l50 C. is discharged into conduit 62 for flow to a first work expander such as turbine 80. The nitrogen gas is expanded therein to an intermediate pressure of about 35 p.s.i.g. and simultaneously cooled to about l81 C. for discharge through conduit 81 to passageway 82 in liquefier 34-. The cold nitrogen gas in passageway 82 flows in countercurrent heat exchange relation with the condensing feed gas in passageways 35 and 36, and is simultaneously partially rewarmed to about 17l C. The latter gas stream is then directed to second Work expander 83 for further expansion to about 10 p.s.i.g. and cooling to about l87 C. The resulting cold, low pressure nitrogen refrigerant gas is discharged into conduit 65 for consecutive flow through the liquefier 34, cold leg 26, forecooler 22, and Warm leg 16 in the previously described manner.

Since the employment of two consecutive work expansion steps offers advantages over the single expansion step embodiment, the question may be posed whether three work expansion steps would not provide even greater efficiencies and economies. The answer to this problem is found by a close inspection of FIG. 3 which is a graph showing the relationship between local temperature differences (AT) between the cooling feed or product fluid and the Warming refrigerant gas in the various heat exchangers, and the enthalpy of the product fluid in that particular locality. It will be noted that generally the transfer of gases from one heat exchanger to the succeeding heat exchanger is indicated by an abrupt change in AT but that in the liquefier, three such changes occur, indicating the desuperheating condensing and subcooling of the product fluid. It can be readily seen that the local AT at the warm end of the cold leg is on the order of 13 C. in the one-stage embodiment, and that the forecooler carries only a small percentage of the total refrigeration load. In the two work expansion stage embodiment, approximately the same amount of low temperature refrigeration is generated. However, a thermal pinch point occurs at the Warm end of the cold leg heat exchanger with a local AT of only about 1 C., and additional inexpensive refrigeration is developed in the forecooler. This ultimately results in about 39% more liquid product than is attainable in the single stage form. In view of the extremely small AT attained at the warm end of the cold leg exchanger, it will be readily apparent that the maximum forecooler refrigeration load has already been achieved with two Work expansion steps, and additional steps would not further increase the product liquefaction capacity.

Another novel feature of the FIG. 2 embodiment is a separator at the cold end of the liquefier for recovering evaporated liquid product and recycling such evaporation gas to the work expanded refrigerant gas stream. It is to be understood, however, that this feature is only suitable Where at least part of the product liquid is throttled from the feed gas pressure to a relatively low pressure for storage purposes, and the throttled part has the same chemical composition as the refrigerant gas. Referring now more specifically to FIG. 2, the nitrogen liquid product in conduit 38 is throttled through control valve 41 to a low pressure such as 10 p.s.i.g., and passed to separator 72 for disengagement of the evaporator gas from the liquid portion. The latter is withdrawn through conduit 76 as low pressure liquid product, and the gas is vented through conduit 84 for juncture with the low pressure work expanded nitrogen refrigerant gas in conduit 65. The composite cold stream is then directed through the heat exchanger series for recovery of sensible refrigeration.

FIGS. 4 and 5 illustrate additional embodiments of the invention which offer significant advantages when the feed and refrigerant gas have the same chemical composition. Instead of employing separate compressors for the two gases, the same compressor may be used to simultaneously pressurize both streams. Alternatively, the streams may be simultaneously compressed to an intermediate pressure in a first compressor, reseparated, and separately compressed in booster compressors. The FIGS. 4 and 5 embodiments will only be specifically described with respect to these features, and are similar to the FIG. 1 system in all other respects.

Referring now to FIG. 4, the low pressure warmed refrigerant gas discharged from the warm end of warm leg 16 into conduit 68 is mixed with the feed gas in conduit 10, and the mixture is passed to compressor at about 8 p.s.i.g. for pressurization therein to an intermediate level, e.g., 98 p.s.i.g. The pressurized mixture is discharged therefrom into conduit 52, and aftercooled in passageway 53 by heat exchange with a colder fluid in thermally associated passageway 54. The aftercooled gas mixture is then further compressed in the booster compressor to a pressure of preferably about 145 p.s.i.g., and discharged therefrom into conduit 56. The further compressed gas mixture is then aftercooled in passageway 57 again to a temperature below about 40 C. by heat exchange with an appropriate fluid such as water in thermally associated passageway 58.

The aftercooled and further compressed gas mixture in conduit 56 is next divided into two parts at the warm end of warm leg heat exchanger 16. One part constitutes the feed gas which is directed to passageway 17 for cooling in warm leg 16 followed by successive cooling in forecooler 22, cold leg 26 and liquefier 34 in the previously described manner. The other part of the compressed gas mixture is directed to passageway 57' as the recycled refrigerant gas.

In FIG. 5, the feed gas and recycled refrigerant gas are both compressed to an intermediate pressure in first or base compressor 51, and aftercooled in passageway 53. However, the aftercooled, partially compressed gas mixture is divided into two portions. One part is directed through booster compressor 55 as the recycling refrigerant gas, and subsequently passed to passageway 57 of warm leg heat exchanger for flow in the previously described manner. The other part of the aftercooled, partially compressed gas mixture is diverted from conduit 52 through branch conduit 90 as the feed gas, and further compressed in separate booster compressor 91 to a pressure of, for example 300 p.s.i.g. The further compressed feed gas is discharged therefrom into conduit 92, aftercooled in passageway 93 by heat exchange with a suitable coolant such as water in thermally associated passageway 94, and introduced to passageway 17 at the Warm end of warm leg heat exchanger 16.

Although preferred embodiments of the invention have been described in detail, it is contemplated that modifications of the process and apparatus may be made and that some features may be employed without others, all within the spirit thereof as set forth herein.

What is claimed is:

l. A refrigeration process for liquefying feed gas by heat exchange with a refrigerant gas, said feed gas and said refrigerant gas both having boiling points below C. at atmospheric pressure, comprising the steps of providing said feed gas and compressing such gas to a pressure of at least 70 p.s.i.g.; partially cooling said feed gas in a first cooling step; further cooling said feed gas in a second cooling step; liquefying and subcooling said feed gas and withdrawing the liquid from such liquefaction step as a liquid product; providing and compressing said refrigerant gas to a pressure of at least 70 p.s.i.g.; after cooling the compressed refrigerant gas to a temperature below about 40 C.; work expanding the compressed and aftercooled refrigerant gas to sufficiently low pressure to develop power and cool the gas to a temperature below the condensation temperature of said feed gas; passing the work expanded refrigerant gas consecutively through the liquefaction, further cooling, and first cooling steps in counter-current heat exchange with the feed stream to effect such cooling and liquefaction thereof; withdrawing the warmed refrigerant gas from the warm end of the first cooling step and recirculating such refrigerant gas to the refrigerant compression step.

2. A process according to claim 1 wherein the refrigerant is nitrogen.

3. A process according to claim 1 wherein the refrigerant is air.

4. A process according to claim 1 wherein the partially cooled feed gas is forecooled by heat exchange with an externally supplied refrigerant before said second cooling step.

5. A process according to claim 1 wherein the refrigerant is nitrogen and the feed gas is oxygen.

6. A process according to claim 1 wherein the refrigerant and feed gas are nitrogen.

7. A process according to claim 1 wherein at least part of the power developed in the work expansion step is transferred directly to the refrigerant compression step at the highest pressure level thereof.

8. A process according to claim 1 wherein said refrigerant gas is successively compressed in a first compression step and a booster compression step, and at least part of the power developed in the work expansion step is transferred directly to said booster compression step.

9. A process according to claim 1 wherein said refrigerant gas is successively compressed in a first compression step to about 100 p.s.i.g., and in a booster compression step to about 145 p.s.i.g., and at least part of the power developed in the work expansion step is transferred directly to said booster compression step.

10. A process according to claim 1 wherein the refrigerant is nitrogen which is compressed to about 145 p.s.i.g. and the feed gas is oxygen which is compressed to about 145 p.s.i.g., and the recirculation ratio is about 8.5 c.f.h. (NTP) nitrogen circulated per 1 c.f.h. (NTP) oxygen liquefied.

11. A process according to claim 1 wherein said compressed and after-cooled refrigerant is further cooled by consecutive passage through the first cooling and further cooling steps before passage to the work expansion step.

12. A process according to claim 1 in which the liquid product is throttled from the feed gas pressure to a relatively low pressure thereby evaporating a portion of such liquid, the evaporated and liquid portions of the throttled fluid are separated, and said evaporated portion is returned to said feed gas for recompression therewith.

13. A process according to claim 1 wherein two separate components form said feed gas.

14. A process according to claim 4 wherein said compressed and aftercooled refrigerant is work expanded to an intermediate pressure, partially warmed by heat exchange with said feed gas in said liquefaction step, and further work expanded to said low pressure.

15. A process according to claim 1 in which at least part of the feed gas and said refrigerant gas have the same chemical composition, the liquid product having said same chemical composition is throttled from the feed gas pressure to a relatively low pressure thereby evaporating a portion of such liquid, the evaporated and liquid portions of the throttled fluid are separated, and said evaporated portion is mixed with said work expanded refrigerant gas for passage through said liquefaction, further cooling and first cooling steps.

16. A process according to claim 1 wherein said feed gas and said refrigerant gas have the same chemical composition and are mixed for the compression to at least 70 p.s.i.g.

17. A process according to claim 1 wherein said feed gas and said refrigerant gas have the same chemical composition, are mixed for compression in a first compression step, separated and further compressed in booster compression steps prior to said partial cooling of said feed gas and aftercooling of said compressed refrigerant.

18. Refrigeration apparatus for liquefying feed gas by heat exchange with a refrigerant gas, said feed gas and said refrigerant both having boiling points below C. at atmospheric pressure, comprising means for providing said refrigerant gas; means for compressing said refrigerant to a pressure of at least 80 p.s.i.g.; means for aftercooling the compressed refrigerant to a temperature below about 40 C.; means for work expanding the compressed and aftercooled refrigerant gas to sufficiently low pressure to develop power and cool the gas to a temperature below the condensation temperature of said feed gas; means for providing said feed gas and compressing such gas to a pressure of at least 70 p.s.i.g.; first heat exchanger means for partially cooling the compressed feed gas; second heat exchanger means for further cooling the partially cooled feed gas; means for liquefying the further cooled feed gas; means for withdrawing the liquefied feed from the liquefier as a product; means for passing the work expanded refrigerant gas consecutively through said liquefier, second heat exchanger means, and first heat exchanger means in countercurrent heat exchange with the feed stream to effect such cooling and liquefaction thereof; means for Withdrawing the warmed refrigerant gas from the warm end of said first heat exchanger means and recirculating such refrigerant to the refrigerant compression means and means for consecutively passing said compressed and aftercooled refrigerant gas through said first and second heat exchanger means for further cooling therein before passage to the work expansion means.

19. Apparatus according to claim 18 wherein means are provided for supplying an external refrigerant, and a forecooler is provided between said first and second heat exchange means for further cooling said partially cooled feed stream, and means for introducing said external refrigerant to said forecooler to effect said further cooling.

20. Apparatus according to claim 18 including means for directly transferring the power developed by the work expander means to the highest pressure level of the refrigerant gas compression means.

21. Apparatus according to claim 18 wherein the refrigerant gas compression means comprise a first compressor and a booster compressor, and means are pro vided for directly transferring the power developed by the work expander means to said booster compressor.

22. Apparatus according to claim 18 including means for mixing said warmed refrigerant gas with said feed gas to form a low pressure gas mixture, whereby a single compressor constitutes said means for compressing said refrigerant and said feed gas to at least 80 p.s.i.

23. Apparatus according to claim 22 wherein means are provided for separating the gas mixture into refrigerant and feed gas fractions, and booster compressors are provided for separately further compressing such fractions.

References Cited in the file of this patent UNITED STATES PATENTS 1,574,119 Seligman Feb. 23, 1926 2,458,894 Collins Ian. 11, 1949 2,494,120 Ferro Jan. 10, 1950 2,495,549 Roberts Jan. 24, 1950 2,496,380 Crawford Feb. 7, 1950 2,552,451 Patterson May 8, 1951 2,705,406 Morrison Apr. 5, 1955 2,762,208 Dennis Sept. 11, 1956 2,764,877 Kohler Oct. 2, 1956 2,823,523 Eakin Feb. 18, 1958 2,909,903 Zimmerman Oct. 27, 1959 2,928,254 Rae Mar. 15, 1960 2,932,173 Mordhorst Apr. 12, 1960 3,058,314 Gardner Oct. 16, 1962 FOREIGN PATENTS 440,702 France July 19, 1912 355,852 Italy Nov. 5, 1936 1,187,120 France Mar. 2, 1959 

1. A REFRIGERATION PROCESS FOR LIQUEFYING FEED GAS BY HEAT EXCHANGE WITH A REFRIGERANT GAS, SAID FEED GAS AND SAID REFRIGRANT GAS BOTH HAVING BOILING POINTS BELOW -80*C. AT ATMOSPHERIC PRESSURE, COMPRISING THE STEPS OF PROVIDING SAID FEED GAS AND COMPRESSING SUCH GAS TO A PRESSURE OF AT LEAST 70 P.S.I.G.; PARTIALLY COOLING SAID FEED GAS IN A FIRST COOLING STEP; FURTHER COOLING SAID FEED GAS IN A SECOND COOLING STEP; LIQUEFYING AND SUBCOOLING SAID FEEDGAS AND WITHDRAWING THE LIQUID FROM SUCH LIQUEFACTION STEP AS A LIQUID PRODUCT; PROVIDIGN AND COMPRESSING SAID REFRIGERANT GAS TO A PRESSURE OF AT LEAST 70 P.S.I.G.; AFTER COOLING THE COMPRESSED REFRIGERANT GAS TO A TEMPERATURE BELOW ABOUT 40*C.; WORK EXPANDING THE COMPRESSED AND AFTERCOOLED REFRIGERANT GAS TO SUFFICIENTLY LOW PRESSURE TO DEVELOP POWER AND COOL THE GAS TO A TEMPERATURE BELOW THE CONDENSATION TEMPERATURE OF SAID FEED GAS; PASSING THE WORK EXPANDED REFRIGERANT GAS CONSECUTIVELY THROUGH TEH LIQUEFACTION, FURTHER COOLING, AND FIRST COOLING STEPS IN COUNTER-CURRENT HEAT EXCHANGE WITH THE FEED STREAM TO EFFECT SUCH COOLIGN AND LIQUEFACTION THEREOF; WITHDRAWING THE WARMED REFRIGERANT GAS FROM THE WARM END OF THE FIRST COOLING STEP AND RECIRCULATING SUCH REFRIGERANT GAS TO THE REFRIGERANT COMPRESSION STEP. 